1,1-Dichloro-1,2,2,2-tetrafluoroethane (also called F114a) is a useful intermediate in the synthesis of 1-chloro-1,2,2,2-tetrafluoroethane, and in particular for that of 1,1,1,2-tetrafluoroethane. The use of which can be envisaged as a substitute for dichlorodifluoromethane.
The production of 1,1-dichloro-1,2,2,2-tetrafluoroethane from 1,1,2-trichloro-1,2,2,-trifluoroethane, hexachloroethane or a mixture of chlorine and tetrachloroethylene by gas-phase processes has already been described in the literature. Thus, for example, in the article by M. Vecchio, et al., Fluorine Chemistry, 4(2), 117-139 (1974) an aluminum fluoride-based catalyst containing small quantities of nickel and/or of chromium is used. The principal disadvantage of this method is its lack of selectivity (joint systematic formation of 1,2-dichloro-1,2,2-tetrafluoroethane, chloropentafluoroethane, and sometimes even hexafluoroethane which has no value) and the risk of the catalyst behaving badly with time. The process according to French Patent No. 1,358,997, which provides for the use of black chrome oxides as catalysts, has the same type of disadvantages.
U.S. Pat. No. 2,748,177 describes in Example 5 a process for the fluorination of 1,1,1-trichloror-2,2,2-trifluoroethane in the gas phase in the presence of a catalyst constituted of aluminum trifluoride. The principal disadvantage of this process is the coproduction of chloropentafluoroethane. According to L. Marangoni, et al., La Chemica e l'Industria, Vol. 64, No. 3, pp. 135-140 (March 1982), a 94.3% yield of 1,1-dichloro-1,2,2,2-tetrafluoroethane could be obtained when fluorinating 1,1,1-trichloro-2,2,2-trifluoroethane in the gas phase using a massive chrome oxide prepared from chrome alum as catalyst. However, the preparation of this catalyst (more particularly its molding) has proved delicate and costly.
Although liquid-phase fluorination in the presence of chlorofluorinated antimony compounds is a known general method, no document describes the use of this method to the selective preparation of 1,1-dichloro-1,2,2,2-tetrafluoroethane. See E. Forche, "Herstellung von Fluorverbindungen", Houben-Weyl 4th Ed.,
Vol V/3, pp. 126-135; U.S. Pat. Nos. 1,934,943; 1,978,840; 2,005,708 and 2,005,710 and French Patent Nos. 720,474; 730,370; 732,320 and 1,166,833.
On the other hand, it is known that preparation of 1,1,1-trichloro-2,2,2-trifluoroethane (F113a) by isomerization of 1,1,2-trichloro-1,2,2-trifluoroethane (F113) only provides pure F113a with great difficulty, but generally leads to a mixture of the two isomers. The proportion of F113 is capable of being up to 50%, and most often is between 1 and 10%. Now, taking account of the proximity of the boiling points of these isomers (47.6.degree. C. for F113 and 46.degree. C. for F113a) their separation by distillation is difficult to envisage on an industrial scale.
The above references are hereby incorporated by reference.